1. Field of the Invention
The present invention relates to a new class of biosensors capable of sensing/monitoring at least one property of a biological system.
More particularly, the present invention relates to a new class of biosensors capable of sensing/monitoring at least one property of a biological system comprising a multi-layered structure having at least one sensing layer and at least one LBL layer, where the sensing layer and the LBL layer may be the same or different and to methods for sensing/monitoring the at least one property of the biological system and where the biosensors may be implantable or merely placed in contact with the biological system.
2. Description of the Related Art
In recent years, there have been extensive research and development efforts aimed at developing bio-sensors that can be reliably used for monitoring and quantifying changes in physiological, biochemical and/or morphological state of biological systems such as individual organs, tissues, or the body as a whole. The development of bio-sensors has been greatly advanced by recent developments in polymer technology and tissue engineering. A number of groups have reported successful deployment of polymer-based sensors for measurements of physiological and/or biochemical indicators such as glucose. The development of biomedical sensors have also benefited significantly from recent developments in the field of nanotechnology and its applications in medicine and biotechnology as well as developments in molecular recognition and molecular targeting techniques that can be incorporated into bio-sensors. The integration of these technologies provides an excellent opportunity for the development of new class of sensors that will utilize the latest developments in the fields of molecular and structural biology, polymer technology and nanotechnology. However, the application of polymer-based sensors in medicine could be limited due to the lack of specificity, lack of sufficient contrast for detection, lack of long term stability and lack of functionality that may limit the use of such sensors in continuous monitoring of biological systems.
When light interacts with a turbid medium such as skin, photons are either absorbed or redistributed in tissue via forward and backward scattering resulting in attenuation of light intensity [16]. The ability of a tissue to absorb and scatter light can be quantified by characterizing its optical properties mainly, the absorption (μa) and scattering (μs) coefficients [17]. These efforts have lead to the development of many promising diagnostic applications for optical spectroscopy and optical imaging in medicine.
The technical challenges associated with the development of a non-invasive sensor led to the development of implantable sensors for minimally invasive monitoring of different biological analytes (predominantly glucose).
Reversible and irreversible changes in the biochemical composition of tissue can lead to detectable changes in tissue properties such as the optical properties of tissue (i.e. optical pathlength) in various regions of optical spectrum (i.e., UV, Visible, Near infrared and infrared). In the near infrared region, changes in tissue scattering are more specifically attributed to the analyte as an osmolyte than changes in tissue absorption spectra. The scattering coefficient, μs, and reduced scattering coefficient, μs′, are dependent on the refractive index (n) mismatch between the extracellular fluid (ECF) and the cellular membranes as well as the morphology and dimensions of cells. In the near infrared spectral range, the index of refraction of the ECF is between about 1.348 and about 1.352, whereas the index of refraction of the cellular membranes and protein aggregates is in the range of about 1.350 to about 1.460. An increase in analyte concentration in the extracellular fluid increases its refractive index. Therefore, adding glucose to blood raises the refractive index of the ECF and consequently decreases the scattering coefficient of the tissue as a whole. This effect has been observed in tissue-simulating phantoms and in vivo using diffuse reflectance measurement systems. In these measurements, the optical signal that was monitored remotely came from photons that were diffusely reflected/backscattered from the scattering centers such as cells within tissue after they have traveled through a multi-layered, highly heterogeneous structure like skin and participated in a large number of scattering events before they could be detected. Thus, in these types of measurements where near-IR light propagation in tissue is dominated by scattering events, the detected glucose-induced changes in optical pathlength (scattering coefficient) of tissue only reflects an average change in the optical pathlength as function of glucose concentration and cannot be resolved spatially to provide a highly sensitive and specific assessment of the dependence of optical pathlength in tissue on glucose concentration. Additional fluctuations in the reported measurements are caused by the inability of these techniques to optically probe a pre-determined region of the tissue for an accurate measurement of analytes in general, and blood glucose in particular, on a reproducible basis.
Approximately, 14 million people in the USA and more than 120 million people all over the world suffer from diabetes mellitus, a chronic systemic metabolic disease. Self-monitoring of blood glucose is the recommended treatment for all insulin dependent diabetic patients.1A In addition, since the announcement of the Diabetes Control and Complications trial results, there is now no question that intensive management of blood sugars is an effective means to prevent or at least slow the progression of diabetic complications once present. Implementation of these intensive management strategies requires accurate and frequent self-monitoring of blood sugars. Unfortunately, it is frequently difficult to obtain the appropriate motivation and dedication on the part of the diabetic patients to successfully implement an intensive program of blood sugar monitoring. Reasons for the lack of compliance are numerous but include the pain associated with obtaining a blood sample and cost. In addition, there are a number of other illnesses that require constant monitoring of important biological intermediates, such as neurotransmitters, nitrogen oxides, hormones, enzymes, pH and other parameters. In each of these illnesses, invasive methods of analysis are typically utilized to monitor and control the illness. Again, these invasive methods involve taking and processing blood and/or other body fluids samples.
In the past two decades, there has been a strong effort towards the developments of noninvasively and minimally invasive techniques for quantifying blood chemicals, particularly glucose, using various optical approaches. These include fluorescence,2A infrared absorption spectroscopy,3A,4A polarimetry,5A and Raman spectroscopy.6A As yet, none of these systems fully meets the expected performance nor have they achieved practical significance. Each system has associated limitations such as: (1) low sensitivity (signal-to-noise ratio) for the glucose concentrations at clinically relevant levels, and/or (2) insufficient specificity for glucose detection.
Current approaches to engineering of implantable sensors are predominantly focused on electrochemical devices. However, the electrochemical mode of monitoring necessitates external wires, which are both uncomfortable for the patient and may result in inflammation of the target tissue. The technical challenges associated with the development of a non-invasive glucose sensor have lead others to propose development of implantable sensors for minimally invasive monitoring of glucose. These sensors are polymer-based and are designed to respond to changes in glucose concentration by either swelling, altering the index of refraction, changing fluorescence characteristics or changing turbidity/optical clarity.7A-10A However, accurate quantitative assessment of changes under in vivo conditions has proven to be a challenge due to lack of optical contrast and/or lack of high specificity. For example, fluorescence spectroscopy has been pursued with the aim to develop an analyte-specific fluorescence probe that can be incorporated into an implantable sensor for remote sensing. However, issues related to the long-term stability of the fluorescence probe and the lack of reversibility of the response has significantly slowed the development of this promising approach.25A 
It is known that glucose can alter optical properties of tissue by reducing light scattering in tissue based on its properties as an osmolyte.11A This effect was demonstrated in vivo using diffuse reflectance measurement.11A However, due to a large number of scattering events that occur in tissue, the detected glucose-induced changes in optical pathlength reflect only an average change in the optical pathlength as a function of glucose concentration and can not be resolved spatially to provide a highly sensitive assessment of the dependence of optical pathlength in tissue on glucose concentration.
Thus, there is a need in the art for new, accurate and reliable implantable or tissue communicating biosensors for non-invasive or substantially non-invasive or minimally invasive monitoring of tissue characteristics and/or bodily fluid characteristics based on polymeric multi-layered composite constructs having properties (physical and/or chemical properties) capable of detection via optical, spectroscopic, optoacoustic, nmr, mri, ultrasonic, or other detection techniques, where a change in response of the biosensor corresponds to a change in concentration of a component of interest in the tissue and/or bodily fluid such as glucose.